The evaporating fuel of vehicles, which often leaks into the air from a fuel tank and a float chamber of a carburetor, contains a large amount of hydrocarbon (HC), which is regarded as one of the causes of air pollution and which often results in waste of fuel. Various techniques are known as means for preventing this occurrence. As a typical technique, there is known an evaporating fuel control apparatus (i.e. an evaporation system) in which, during the operation of the internal combustion engine, evaporative fuel generated in the fuel tank is purged in a canister containing an absorbent such as activated carbon so as to be supplied to the internal combustion engine.
As shown in FIG. 5, this known evaporating fuel control apparatus comprises an air passageway 112 for communicating the inside of a surge tank 108, which is part of an air inlet passageway 106 on the downstream side of a throttle valve 104 in an air inlet system, with a fuel tank 110. A canister 114 is placed midway of the air passageway 112. This canister 114 is adapted to absorbingly hold the evaporating fuel generated in the fuel tank 110 during stopping of the internal combustion engine 102, and purging such held evaporating fuel by the introduction of fresh air during the operation of the internal combustion engine 2. With this feature, the air passageway 112 is divided into an evaporation passageway 116 between the fuel tank 110 and the canister 114, and a purge passageway 118 between the canister 114 and the air inlet passageway 106.
Midway of the evaporation passageway 116, there are provided a pressure sensor 122 through a detecting pressure introduction passageway 120 and a pressure control valve (TPCV) 124 arranged in this order from the fuel tank side. A purge valve (duty solenoid valve) 126 is placed in the purge passageway 118.
One end of a pressure passageway 128 is connected to the pressure control valve 124. The other end of the pressure passageway 128 is in communication with the inside of the surge tank 108. A three-way cut valve 130 is positioned midway of the pressure passageway 128.
One end of an air introduction passageway 132 is connected to the canister 114, and the other end is in communication with an air cleaner 134. Midway of the air introduction passageway 132, there is provided an air cut valve 136 adapted to cut the supply of the air to the canister 114.
The pressure sensor 122, the purge valve 126 and the air cut valve 136 are in communication with a control means 138.
With this feature, in order to check whether or not the apparatus is in a normal working condition, in the evaporating fuel control apparatus of FIG. 5, as shown in FIG. 6, during the operation of the internal combustion engine 102 a negative pressure temporarily acts on the whole air passageway 112 including the fuel tank 110 and thereafter the purge valve 126 and air cut valve 136 are maintained in their closed positions, thereby cutting the communication between the air and all the passageways. Then, the variation level of the negative pressure held by the whole air passageway 112 is detected. That is, it is judged whether or not air-tightness, etc. are maintained, such as, there being a leak when the rising of pressure is rapid, for example.
A failure checking apparatus of an evaporating fuel control apparatus of the type mentioned above is disclosed, for example, in Japanese Laid-Open Patent Application No. Hei 4-362264. The apparatus disclosed in this Laid-Open Publication carries out the detection of a failure as follows. Immediately after the start of the internal combustion engine and when the temperature of the engine is equal to or lower than a predetermined level, a failure checking valve is opened or closed and a purging control valve is opened to introduce a negative pressure in an air inlet tube, and then the purging control valve is opened and maintained in that condition for a predetermined time period, so that a failure can be checked with reference to the variation of the pressure level within the predetermined time period. In this way, it is not only a large amount of leakage of vapor from the whole evaporation purge system including the purge passageway but also a small amount of leakage of vapor, that can be detected without a misjudgment.
However, in the above known evaporating fuel control apparatus, when the air cut valve 136 as placed in the air introduction passageway between the canister 114 and air cleaner 134 is accidentally closed, a negative pressure acts directly on the inside of the fuel tank 110 through the purge valve 126 during the operation of the internal combustion engine. The result is a possibility that the fuel in the fuel tank is drawn directly to the inlet system through the air passageway, depending on the amount of the negative pressure. There is another possibility that when there is a predetermined condition for closing the purge valve, the fuel tank is hermetically closed and in addition, the evaporating fuel in the canister is not purged. In such a case, the pressure in the fuel tank becomes abnormal with the results that the operating efficiency is deteriorated and the evaporating fuel is sometimes leaked outside. Therefore, improvements are demanded.
Therefore, according to a first aspect of the present invention, in order to obviate the above-mentioned inconveniences, there is provided an evaporating fuel control apparatus for an internal combustion engine comprising a canister place midway of an air passageway for intercommunicating the inside of a fuel tank and an air inlet passageway of an air inlet system of the internal combustion engine and adapted to absorbingly hold evaporated fuel as generated in the fuel tank during stopping of the internal combustion engine, and purging such held evaporated fuel by introduction of fresh air so as to be supplied to the air inlet passageway during the operation of the internal combustion engine, a purge valve placed midway of the air passageway between the canister and the air inlet passageway and adapted to control the amount of evaporated fuel supplied to the air inlet passageway depending on the operating condition of the internal combustion engine, and an air cut valve for feeding and stopping the supply of air to the canister; the evaporating fuel control apparatus further comprising a two-way cut valve adapted to prevent the internal pressure of the fuel tank from becoming abnormal when the air cut valve is closed due to a failure.
According to a second aspect of the invention, the evaporating fuel control apparatus for an internal combustion engine, as aforesaid, further comprises an air communication passageway connected midway of the air passageway between the canister and the purge valve, and the two-way cut valve is placed midway of the air communication passageway.
According to a third aspect of the invention, the evaporating fuel control apparatus for an internal combustion engine, as aforesaid, further comprises an air communication passageway connected midway of an air introduction passageway between the canister and the air cut valve, and the two-way cut valve is placed in the air communication passageway.
According to a fourth aspect of the invention, said evaporating fuel control apparatus for an internal combustion engine, as aforesaid, further comprises the two-way cut valve including a positive pressure check valve and a negative pressure check valve, mounted on a cap of said fuel tank and adapted to maintain a predetermined level of internal pressure in said fuel tank.
According to the construction of the present invention, in case there is a possibility that the pressure in the fuel tank becomes abnormal due to the air cut valve being subjected to a failure by one reason or another, the two-way cut valve is actuated to introduce the air to maintain the predetermined level of pressure in the fuel tank. Accordingly, the fuel in the fuel tank is prevented from being supplied directly to the air inlet passageway through the air passageway so that the operating efficiency can be improved, and the evaporating fuel can be prevented from leaking outside.